1. Technical Field
The present invention relates to receivers, such as measuring receivers used in vector network analyzers (VNAs) and in particular to calibration methods for RF receiver gain ranging systems.
2. Related Art
Receivers, particularly measuring receivers used in VNAs, can use radio frequency (RF) gain ranging to optimize the trade-off between compression and noise floor. In contrast to the more common intermediate frequency (IF) gain ranging, RF gain ranging is often used when the receiver would otherwise be very noise figure-challenged. In such a structure, a variable gain stage is placed near the front-end of the receiver prior to down-conversion.
Measuring systems generally rely on calibration, to ensure accurate measurement results. Receivers, such as those described above which can operate in multiple gain states, need to be calibrated across those gain states. A typical calibration technique includes injecting a training signal and then measuring the receiver output in two gain states. The ratio of the output in the first state to the output in the second state can then be calculated. Generally, phase is not preserved in this technique or it requires a more complex synchronization scheme between the training signal and the receiver. This approach is thus inadequate for most vector measurement applications.
Another typical technique uses the ratio of a test signal (TEST), output by a particular receiver, to the input signal (REF), received by the device under test, as the calibration value since it is often used anyway in correction algorithms. This term is commonly called transmission tracking (etf) and is used as a normalizer when applying calibrations. Different values of this normalizer are stored for the different gain states and the appropriate one is chosen at measurement time based on the current gain state. As shown below, the ‘˜’ denotes a measurement taken while the instrument is in gain state 2 (state2).
      etf          state      ⁢                          ⁢      1        =            (              TEST        REF            )              state      ⁢                          ⁢      1      
      etf          state      ⁢                          ⁢      2        =            (                        TEST          ~                          REF          ~                    )              state      ⁢                          ⁢      2      
A problem is that the state of the drive system during these calibration steps in general will not be the same as the state during the actual measurement of a device under test (DUT). In general, the match of the drive system will be a function of that state and thus mismatch ripple can change between calibration and measurement. Among other issues, the requirement not to overload the receiver during calibration will often result in some extreme states of this drive system and can lead to larger errors.